1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus which has a plurality of image carriers and is capable of superimposing and forming images (e.g., images for each color), which are formed on each of image carriers, on a recording medium.
2. Description of the Related Art
An image forming apparatus has been proposed which has a plurality of recording units (for example, laser beam printers) in which each recording unit irradiates a laser beam modulated in accordance with record information onto a photosensitive drum, develops an electrostatic latent image on the photosensitive drum by an electrophotographic process, and transfers the image to a transfer paper. Each color image is transferred and superimposed during transport of the transfer paper through the recording units by means of a transfer belt, thereby making it possible to form a multi-color image.
In this type of image forming apparatus, if there are mechanical mounting errors between the photosensitive drums, optical path length errors between the light beams, or changes in the optical path between the light beams, the images for each color formed by forming electrostatic latent images on the various photosensitive drums, and then developing and transferring them on the recording paper on the transfer belt, will not be registered correctly. For this reason, a pattern image for registration correction is read by an image sensor such as a CCD sensor, and the pattern image is transferred onto a transfer belt from the various photosensitive drums. The position of the registration correction pattern for each color is determined on the basis of the density values of the read data. Registration deviations on the photosensitive drum respectively corresponding to each of the colors are detected on the basis of the thus determined position. Image signals to be recorded are subjected to electrical corrections in accordance with the detected deviations, and/or a reflection mirror disposed in the optical path of the light beams is driven to correct changes in the optical path length or the optical path.
The image processing section of the image forming apparatus may read out data which has been written in a memory in block units (for example, block unit compression). The relative position of the blocks to be read is not varied in accordance with the amount of deviation in a plurality of image forming means.
However, since registration corrections are performed at set timings (such as at fixed time intervals or when a fixed number of images have been formed), the above-described image forming apparatus has a problem in that it does not account for cases in which, for example, the temperature of the machine increases when the machine is powered on or when the machine is moved. Since changes in registration deviations are large in such conditions, temporary registration variations occur.
In a case in which pattern images for registration correction, which are transferred on a transfer belt from the respective corresponding photosensitive drums, are read by a CCD sensor or the like, the read pattern images are stored in a memory, and the positions of the registration correction patterns for each color are determined on the basis of the density values of the read data in accordance with pattern image data read out sequentially from the memory. In such a case, when the transferred registration correction pattern image cannot be formed clearly due to changes in the environment, or changes in the process conditions such as latent imaging, development or transfer of images, or when an image is formed on scratches or contaminants on the transfer belt, the central position of the registration correction image is erroneously computed on the basis of the read data. As a consequence, an error occurs in the computation of registration deviations of each color, causing the registration to deviate.
For example, when an image is formed normally on the transfer belt, the histogram data of the density additions regarding a pattern image in the main-scanning and sub-scanning directions is distributed as shown in FIG. 8. The position of the maximum value of the histogram data matches the central position of the histogram, making it possible to easily compute the central value. However, when the transfer conditions vary as shown in FIG. 30, for example, when data is lost during transfer, causing the density in the central portion to be higher than that in the edge portion of the image pattern, or when a scratch is present on the transfer belt as shown in FIG. 31, the maximum value of the histogram does not match the central value of the image pattern. Therefore, the central position of the registration correction image is erroneously computed on the basis of the read data.
In addition, when data written in a memory in block units is read out in block units by the image processing section of the above-described image forming apparatus, and when the image forming section has a plurality of image forming means as described above, registration deviations cannot be adjusted in pixel units since the registration deviations are only adjusted in block units.